Satellite-based positioning systems include constellations of earth orbiting satellites that constantly transmit orbit information and ranging signals to receivers. An example of a satellite-based positioning system is the Global Positioning System (GPS), which includes a constellation of earth orbiting satellites, also referred to as GPS satellites, satellite vehicles, or space vehicles. The GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to the earth. The satellite signal information is received by GPS receivers which can be in portable or mobile units, or in fixed positions on base stations and/or servers.
The GPS receiver uses the satellite signal information to calculate the receiver's precise location. Generally the GPS receiver compares the time GPS signals or satellite signals were transmitted by a satellite with the time of receipt of that signal at the receiver. This time difference between satellite signal reception and transmission provides the receiver with information as to the range of the receiver from the transmitting satellite. Using pseudo-range measurements (pseudo because the range information is offset by an amount proportional to the offset between GPS satellite clock and receiver clock) from a number of additional satellites, the receiver can determine its position. The GPS receiver uses received signals from at least four satellites to calculate three-dimensional position (latitude, longitude, and altitude), or at least three satellites to calculate two-dimensional position (if altitude is known).
As GPS technology becomes more economical and compact it is becoming ever more common in consumer applications. For example, GPS systems are used for navigation in general aviation and commercial aircraft as well as by professional and recreational boaters. Other popular consumer uses of GPS include use in automobile navigation systems, construction equipment, and farm machinery as well as use by hikers, mountain bikers, and skiers, to name a few. Further, many location-based services are now available, such as asset tracking, turn-by-turn routing, and friend finding. Because GPS technology has so many consumer applications, it is finding increased popularity as an additional application hosted by a variety of portable electronic devices like personal digital assistants (PDAs), cellular telephones, and personal computers (PCs), to name a few. The popularity of GPS technology with consumers has resulted in an increased reliance on the position information provided to the consumer by GPS which, in turn, has resulted in a desire for GPS systems that provide reliable position information even when the GPS system is operating under less-than-ideal conditions.
The GPS satellite signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not get through most solid objects such as buildings and mountains. Generally, then, GPS receivers are usable everywhere except where it is impossible to receive an adequate satellite signal such as inside some buildings, in caves and other subterranean locations, and underwater. A GPS receiver, when determining position information, typically relies on information from the satellite signal, the absence of which makes position determination impossible. This satellite signal information includes a pseudorandom code along with ephemeris and almanac data to the receivers. The pseudorandom code is a code that identifies the satellite that is transmitting the corresponding signal and also helps the receiver to make ranging measurements. The almanac data tells the GPS receiver where each GPS satellite of the constellation should be at any time over a wide time interval that spans a few days or weeks. The ephemeris data does the same thing but much more accurately though over a much shorter time interval of a few hours.
The broadcast ephemeris data, which is continuously transmitted by each satellite, contains important information about the orbit of the satellite, and time of validity of this orbit information. In particular, the broadcast ephemeris data of a GPS satellite predicts the satellite's state over a future interval of approximately four hours. The state prediction includes predictions of satellite position, velocity, clock bias, and clock drift. More particularly, the broadcast ephemeris data describe a Keplerian element ellipse with additional corrections that then allow the satellite's position to be calculated in an Earth-centered, Earth-fixed (ECEF) set of rectangular coordinates at any time during the period of validity of the broadcast ephemeris data. Typically, the broadcast ephemeris data is essential for determining a position.
Considering that the broadcast ephemeris data is only valid for a four hour interval and is essential for position determination, a GPS receiver is required to collect new broadcast ephemeris data at such time as the receiver needs to compute the satellite state when the validity time for the previously-collected broadcast ephemeris data has expired. The new broadcast ephemeris data can be collected either as direct broadcast from a GPS satellite or re-transmitted from a server. However, there are situations under which it is not possible to collect new broadcast ephemeris data from GPS satellites or from a server. As an example of situations in which new broadcast ephemeris data cannot be collected, a low signal strength of the satellite signals can prevent decoding/demodulating of the ephemeris data from the received satellite signal, the client can be out of coverage range of the server, and/or the server can be unavailable for a number of reasons, to name a few. When new broadcast ephemeris data is not available, the GPS receiver is typically unable to provide position information.
Furthermore, even when the GPS receiver is in a position from which it can receive the broadcast ephemeris information from a GPS satellite and/or server and properly decode the signal, the process of receiving and decoding adds substantially to the processing time. This additional processing time directly increases the time-to-first-fix (TTFF) while increasing the power usage of the receiver. Both an increase in the TTFF and the power usage can be unacceptable to a user depending on the use being made of the receiver and power capabilities of the receiver (for example, a GPS receiver hosted on a client device like a cellular telephone would have stricter power use constraints). As a result of the increased use of GPS in consumer devices, and the increased reliance on the information provided by such devices, it is desirable to reduce the number of situations in which the GPS receiver cannot provide position information and/or cannot provide position in a time and power efficient manner.